The proposed study will rigorously test the efficacy of an innovative, couples-based HIV/STI risk reduction intervention (CHSR) to decrease new cases of HIV and Hepatitis C (HCV) and incidence of sexually transmitted infections (STIs), as well as to reduce unsafe injection practices and increase condom use among injecting drug users (IDUs) and their heterosexual, intimate partners in Shu, Kazakhstan. Central Asia has experienced one of the fastest growing HIV/AIDS epidemics due to a sharp increase in injection drug use. This investigative team from the Social Intervention Group at Columbia University and the Shu Policlinic Needle Exchange Program in Kazakhstan completed a pilot randomized controlled trial of the proposed CHSR among 40 IDUs and their primary, heterosexual partners (N=80) in Shu, Kazakhstan. Compared to participants assigned to the wellness promotion comparison condition (WP) (n=40), CHSR participants (n=40) reported significantly fewer unsafe injection behaviors and greater condom use from baseline to the 3-month follow-up. For the proposed study, the participants will be 400 IDUs and their heterosexual, intimate partners, who meet eligibility criteria (e.g., at least one partner reports unsafe injection and unprotected sexual intercourse in the past 30 days). Participants will be recruited from the Shu Policlinic needle exchange program and through street outreach in Shu. Shu is located along a major drug trafficking route and has an estimated 4,500 IDUs out of a population of 35,000. In the proposed study, 400 couples will be randomized to one of two interventions: a 5-session couples-based HIV/STI risk reduction intervention or a 5- session couples-based wellness promotion intervention, which will serve as a comparison condition. Participants will be assessed with repeated measures at baseline, 3-, 6- and 12-months post-intervention. The primary behavioral outcomes are self- reported proportion of injection acts in which needles or syringes are shared in the past 30 days and proportion of condom-protected acts of sexual intercourse in the past 30 days. The primary biological outcomes are the rate of new HIV cases and new HCV cases, as well as the incidence of Chlamydia, gonorrhea, and syphilis over the 12-month post-intervention period. The proposed study will advance the understanding of HIV/HCV/STI risk reduction among a population of IDUs and their heterosexual partners, and thereby help to stem the rising epidemic of HIV, HCV, and STIs in Kazakhstan and Central Asia. PUBLIC HEALTH RELEVANCE The proposed study addresses a significant public health threat of HIV, HCV and other STIs among a very high risk population of active IDUs and their sexual partners in Kazakhstan - a region that is experiencing one of the fastest rising HIV epidemics in the world. There is a race to develop and implement effective HIV preventive interventions for IDUs and their sexual partners to stem the spread of HIV, HCV and other STIs in Shu and other Central Asian towns along drug trafficking routes.